The present invention relates to a blend composition of melt-processible tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer with polytetrafluoroethylene having superior resistance to gas permeability and mechanical durability.
Melt-processible polymers comprising tetrafluoroethylene and perfluoro(alkyl vinyl ether), commonly known as PFA, have excellent heat resistance and inertness, i.e. chemical stability, making this copolymer highly useful in the form of melt-fabricated articles such as vessels and tubes for handling and transporting liquid chemicals or ultra-pure water used in semiconductor manufacture. U.S. Pat. No. 5,473,018 discloses the addition of low molecular weight polytetrafluorethylene (PTFE) to PFA to increase the surface smoothness of the melt-fabricated articles so as to minimize adhesion of the liquids handled by the molded PFA articles to avoid contamination of the semiconductor manufacturing process.
Articles melt-fabricated from PFA and used for handling of liquids also need to be gas impermeable and durable so as to withstand the repeated flexing encountered in use. While the problem of improved surface smoothness is solved by the invention disclosed in U.S. Pat. No. 5,473,018, information is not provided therein on gas permeability. In addition, the effect of the adding the low molecular weight PTFE to PFA as disclosed in the ""018 patent is generally to decrease the flexibility of the resultant PFA composition, as characterized by decreased folding endurance. In Example 7, low molecular weight PTFE contents of 5 wt % and above decrease the folding endurance as compared to the article made entirely from PFA (Table 3). In Tables 4 and 5, the reduction in folding endurance occurs at even lower contents of low molecular weight PTFE.
The problem arises as to how to increase the folding endurance of PFA and how to increase the gas impermeability of the PFA as well.
It has been discovered that addition of at least about 5 wt % low molecular weight polytetrafluoroethylene (PTFE) to PFA increases the gas impermeability of the PFA and articles melt-fabricated therefrom. In accordance with the Examples in the ""018 patent, however, this amount of low molecular weight PTFE additive to the PFA has an adverse affect on folding endurance. It has further been discovered that certain PFA/low molecular weight PTFE compositions provide both improved gas impermeability and increased folding endurance.
Thus the present invention is a composition comprising a copolymer of tetrafluoroethylene and perfluoro(alkyl vinyl ether) (PFA) and about 5 to about 30 wt % of low molecular weight PTFE, based on the combined weight of the PFA and low molecular weight PTFE, wherein the low molecular weight of the PTFE is characterized by a heat of crystallization of at least about 50 J/g and wherein the perfluoro(alkyl vinyl ether) content of the PFA is at least about 4 wt %, based on the total weight of the copolymer, and the PFA has a melt flow rate (MFR) of no more than about 4 g/10 min.
When the low molecular weight PTFE content is less than 5 wt % and/or the comonomer content is less than about 4 wt % and/or the MFR of the PFA is greater than about 4 g/10 min, the combination of improved gas impermeability and increased folding endurance is not obtained. It is especially surprising that the increased amount of low molecular weight PTFE as compared to the amounts used in the ""018 patent, which did adversely affect folding endurance, improves folding endurance in the present invention.
In the present invention, a tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer is used. General formulas for the perfluoro(alkyl vinyl ether) are given in (1) and (2).
CF2xe2x95x90CF(OCF2CF(CF3))nxe2x80x94Oxe2x80x94(CF2)mCF2Xxe2x80x83xe2x80x83(1) 
X is H or F; n is an integer of 0 to 4; m is an integer of 0 to 7.
CF2xe2x95x90CF(OCF2CF(CF3))qxe2x80x94Oxe2x80x94CF2CF2CF(CF3)2xe2x80x83xe2x80x83(2) 
q is an integer of 0 to 3.
Perfluoro(propyl vinyl ether) (PPVE) and perfluoro(ethyl vinyl ether) (PEVE) are preferred perfluoro(alkyl vinyl ethers). Especially a copolymer with perfluoro(ethyl vinyl ether) exhibits a great improvement effect on mechanical durability by the addition of low molecular weight PTFE, which is described later; hence it is most preferable.
In the present invention, the perfluoro(alkyl vinyl ether) content of the copolymer is from about 4 wt % or greater, preferably about 5 to about 20 wt %. When the content is less than 4 wt %, mechanical durability is unsatisfactory, whereas when it is greater than about 20 wt % the service temperature of articles made from the polymer blend is too low. Furthermore, the copolymer of the present invention has a melt flow rate (MFR) of about 0.1 to about 4 g/10 min, preferably about 0.5 to about 3 g/10 min at 372xc2x11xc2x0 C. If the MFR exceeds about 4 g/10 min, the blend with low molecular weight PTFE tends to have reduced mechanical durability. When the MFR is below about 0.1 g/10 min melt-processing becomes difficult.
In the present invention, the low molecular weight PTFE added to the above-mentioned copolymer is a tetrafluoroethylene homopolymer or a modified PTFE containing a very small amount, 1 wt % or less, of a comonomer, for example, hexafluoropropylene, perfluoro(alkyl vinyl ether), fluoroalkylethylene or chlorotrifluoroethylene. The low molecular weight of the low molecular weight PTFE is characterized by high crystallinity with a heat of crystallization of about 50 J/g or greater. A typical range of heat of crystallization is from about 50 to about 90 J/g. The low molecular weight PTFE can be made directly from the polymerization of tetrafluoroethylene in the presence of a chain transfer agent. It can also be made by irradiating or by pyrolyzing high molecular weight PTFE of the kind known as xe2x80x9cmolding powderxe2x80x9d (also known as xe2x80x9cgranularxe2x80x9d) or xe2x80x9cfine powderxe2x80x9d, or its moldings made therefrom. The low molecular weight PTFE made by irradiation is preferred. The term xe2x80x9clow molecular weightxe2x80x9d as it applied to PTFE is used in contrast to the very high molecular weights of molding powder or fine powder, which are characterized by a much lower heat of crystallization, e.g. less than about 35 J/g. The heat of crystallization is generally used to characterize the low molecular weight PTFE (sometimes referred to as micropowders, as in ASTM D 5675).
PTFE having a heat of crystallization of less than about 50 J/g, for example, PTFE xe2x80x9cmolding powderxe2x80x9d or xe2x80x9cfine powderxe2x80x9d, is not a satisfactory component of the blend compositions of this invention because it confers unacceptably high viscosity on the blends, which make melt processing difficult.
The optimum amount of low molecular weight PTFE in the composition of the present invention varies depending on the perfluoro(alkyl vinyl ether) content the copolymers. However, it is in the range of about 5 to about 30 wt %. If the content is less than about 5 wt %, a significant improvement in gas permeability resistance is not realized, whereas if it is greater than about 30 wt % mechanical durability is reduced.
In the present invention, there is no restriction on the process for mixing low molecular weight PTFE with the copolymer. Melt-blending, dry blending, and wet blending can be utilized. Also, the low molecular weight PTFE particles can be dispersed in the polymerization kettle in advance in the polymerization to make the PFA copolymer. It is also possible to first polymerize low molecular weight PTFE in the polymerization kettle and thereafter to add tetrafluoroethylene and perfluoro(alkyl vinyl ether) to make the PFA. Since the low molecular weight PTFE used in the present invention is compatible with PFA in the molten state, it disperses readily in the copolymer during melt-blending or melt-extrusion, giving a uniform composition that exhibits the improved properties of the present invention. Therefore there is no restriction on the particle size of low molecular weight PTFE to be mixed, but usually fine particles of 0.05 to 1 xcexcm in average particle size or a powder of several micrometers to several tens of micrometers is used. A typical range is about 0.01 to about 100 xcexcm, preferably about 0.05 to about 50 xcexcm, more preferably about 0.05 to about 25 xcexcm.